For both these two typical cases i.e., systems with random site energies
and samples with external electric field, one cannot find the existence of
mobility edge separating the conducting states from the non-conducting
region, since all the energy levels are localized. Therefore, for these
models no long-range transport will be obtained. However, a large number
of physically relevant models are available which exhibit mobility edges
at some typical energy values. For example, a 1D tight-binding (TB) chain
composed of two uncorrelated random atomic sites exhibits extended energy
eigenstates for some specific electron energies over the entire material
under a suitable alignment of these random sites as originally explored
by Dunlap et al. [1]}. In this case, a short range positional
correlation among the atomic sites is established. On the other hand,
in quasi-periodic Aubry-Andre model [2]}, where long range positional
correlation is found to exist between the atomic sites in 1D geometry,
conducting energy levels are also noticed. All these special classes of
materials, the so-called correlated disordered materials, have provided
a new turn in the localization phenomena.
